1. Field of the Invention
The present invention relates to an oil dashpot and a method of producing the oil dashpot for use in the electromagnetic tripping apparatus of a circuit breaker, such as for example, a molded-case circuit breaker.
2. Description of the related Art
FIG. 7 is a longitudinal sectional view showing the configuration of a conventional electromagnetic tripping apparatus having an oil dashpot. In the drawing, the reference numeral 1 designates an L-shaped yoke; 2, an armature rotatably supported on one leg of the L-shaped yoke 2; 3, a return spring for holding the armature 2 in an illustrated position; 4, an oil dashpot fixed on the other leg of the yoke 1; and 5, a tripping coil provided so as to surround the oil dashpot 4 and connected to an electrical current path of a circuit breaker (not shown).
The oil dashpot 4 is constituted by a cylinder 6 of a non-magnetic material, such as a copper alloy, and a plunger 7 of a magnetic material provided within the cylinder 6. Cylinder 6 is filled with oil, such as for example silicone oil, which acts as a brake to motion of plunger 7 within the cylinder. Compression spring 8 urges the plunger 7 against a bottom portion of the cylinder 6, and a pole piece 9 of a magnetic material seals an open top portion of the cylinder 6. A flange 6a is integrally formed at the open top portion of the cylinder 6. The pole piece 9 has a large diameter portion 9a which contacts with the flange 6a and a small diameter portion 9b which fits into a body portion 6b of the cylinder 6. Pole piece 9 and cylinder 6 are typically joined by soldering or resistance welding the large diameter portion 9a to the flange 6a.
In such a configuration, when a current having at least a rated value flows into the tripping coil 5, the plunger 7 is gradually attracted toward the pole piece 9 by a resulting electromagnetic force. The compression spring 8 opposes the movement of plunger 7 towards the pole piece 9. When the plunger 7 abuts on the pole piece 9, the magnetic reluctance of a magnetic circuit constituted by the yoke 1, the armature 2, the pole piece 9, and the plunger 7 is reduced such that the armature 2 is attracted toward the pole piece 9. The armature 2 acts on a tripping mechanism (not shown) which trips a circuit breaker. The movement to plunger 7 receives a braking force form the silicone oil within the cylinder 6. This braking effect establishes a delay period between the time current flows through tripping coil 5 and the time the circuit breaker is tripped. Once the circuit breaker is tripped, the plunger 7 gradually separates from the pole piece 9 in response to the force of the compression spring 8 and returns to the state illustrated in FIG. 7.
The pole piece 9 in the conventional oil dashpot 4 is produced by cutting one end of a metal rod having a diameter equal to the large diameter portion 9a down to the small-diameter portion 9b. The metal rod used to produce the pole piece 9 is typically a machinable steel having a good cutting efficiency. Accordingly, elements such as sulfur (S), lead (Pb), and the like are added to the machinable steel to reduce the ductility of the material and to thereby improve the machining property of the steel rod. However, since these elements are distributed fibrously in the longitudinal direction of the steel rod, the rod generally lacks strength against a longitudinal exerted external force.
Therefore, the conventional pole piece 9 produced from machinable steel may fracture if a sufficiently great force acts longitudinally on the pole piece 9. Unfortunately, during the process of inserting the pole piece 9 to seal the cylinder 6 may exert just such a longitudinal force on pole piece 9. A possible result being the formation of a fracture in the root portion of the small-diameter portion 9b. As a result, a fractured pole piece 9 may allow silicon oil to leak from the cylinder. Of further note, the process of cutting the rod down to the small diameter portion 9b requires a large number of steps and is therefore very costly.
A similar problem exists in the case where an annular welding protrusion is formed on the large diameter portion 9a in order to improve the joining property of the pole piece 9 when it is resistance-welded to the cylinder 6. (See Japanese Utility Model Post-Examination Publication No. Sho-60-15292.) A fracture may be generated in the root portion of the small diameter portion 9b in the same manner as in the previous case when the welding protrusion is deformed and melted during the welding process.
Conventional techniques of joining the pole piece 9 and the cylinder 6, compress various soldering methods for soldering the periphery of the large diameter portion 9a to the flange 6a of the cylinder. These methods generally address the problem of faulty sealing as a result of uneven imperfections or pinholes in the two joined surfaces. For example, a method has been proposed wherein cylinder 6 is solder-plated and pole piece 9 is heated while being urged against cylinder 6 in order to join the pole piece 9 to the cylinder 6. (See Japanese Utility Model Unexamined Publication No. Sho-62-131346.) This method, however, creates further problems. For example, during the heating process the melted solder may migrate along the inner wall of cylinder 6 and mix with the silicone oil. Such a mixture would change the response delay the oil provides for the plunger and thereby change the time constant for the electromagnetic tripping mechanism.
Another conventional technique used to join the pole piece 9 with the large diameter portion 6a of the cylinder is the method of resistance-welding. This method often causes expulsion and surface flush while a welding protrusion is melted. In turn, these effects create a gap in the junction between the two joined surfaces and allow silicone oil to leak.